1. Related application:
The present application is related to the subject matter of a co-pending application of the same inventors entitled "Apparatus For Impedance Measurement Of Body Tissue" filed simultaneously herewith, Ser. No. 061,549.
2. Field of the Invention
The present invention is directed to devices for making an impedance measurement of body tissues, and in particular to such devices wherein an electrical signal is impressed on the body tissue and an impedance signal is acquired therefrom.
3. Description of the Prior Art
Devices for making an impedance measurement of selected body tissue are known in the art which have a signal source which impresses an electrical signal on the body tissue, means for acquiring an impedance signal from the body tissue dependent on the impressed electrical signal, and means for evaluating the impedance signal. A device which includes an evaluation means which filters out only higher frequency signal components from the impedance signal is described in U.S. Pat. No. 4,303,045. This known device is used in conjunction with frequency-controlled heart pacemakers.
Another apparatus of this type is known from U.S. Pat. No. 3,532,086, wherein an evaluation means filters out both lowfrequency and higher-frequency signal components. This device is used for identifying the degree of blood loss during an operation. The low-frequency component is a measure of the blood volume of the patient.
Heretofore, impedance measurements in body tissue (including blood) served the purpose of identifying mechanical volume changes of the body, for example, the stroke volume of the heart, or the volume changes resulting from thorax movement during respiration. Such changes in impedance are used, for example, to control the frequency of the stimulation pulses of a heart pacemaker. The basis of such impedance measurements is the following known physical relationship: ##EQU1## wherein R is the impedance, .sigma..sub.R is the conductance of the tissue (1/.OMEGA.-cm), and K.sub.1 =l/F is a value (cm.sup.-1) proportional to the line distance with l being the effective electrode spacing (cm) and F being the effective line cross-section (cm.sup.2) between the electrodes.
Changes in the impedance (.DELTA.R) are a measure of the patient respiration and a measure for the stroke volume of the heart. The measurement of periodic impedance fluctuation essentially covers changes .DELTA.l from a nominal value for l, or .DELTA.F from a nominal value for F with respect to the line distance, and thus covers the changes .DELTA.K.sub.1. Changes in the impedance are also
dependent on changes of the conductance (.DELTA..sigma..sub.R) because ##EQU2## applies in accord with the first-stated relationship. Following therefrom, when filtering out the higher frequency components from the impedance signal, is the relationship: ##EQU3## i.e., the impedance changes inversely with respect to the conductance. This causes undesired falsifications of the measured value in conventional impedance measuring devices.